This invention relates to a method for conducting a miscible carbon dioxide and hydrocarbon solvent flood in a steeply dipping reservoir wherein the concentration of hydrocarbon solvent in the carbon dioxide and solvent mixture is decreased as the flood front moves downdip into reservoir areas of higher formation pressure and temperature.
One popular method for enhanced oil recovery is that of miscible flooding wherein a solvent such as carbon dioxide is injected into the reservoir to increase recovery. For any substantial recovery to occur, the carbon dioxide must be at least partially miscible with the reservoir hydrocarbons. Generally, the greater the miscibility of the carbon dioxide, the greater the hydrocarbon recovery. In terms of cost and availability, carbon dioxide is one of the most promising solvents for miscible flooding. Unfortunately, carbon dioxide is frequently not miscible with hydrocarbons in many reservoirs. Thus, methods of mixing other solvents with the carbon dioxide have been developed. The most popular additives to carbon dioxide for creating a miscible flooding system have been various light hydrocarbon solvents, particularly paraffins in the C.sub.2 to C.sub.6 range. Non-hydrocarbons such as hydrogen sulfide may also be added to aid miscibility.
U.S. Pat. No. 2,875,830 discloses an enhanced oil recovery method if injecting a mixture of carbon dioxide and light hydrocarbons to increase recovery. The patent discloses the use of light hydrocarbons in the range of ethane to decane. A second reference, U.S. Pat. No. 3,811,503 emphasizes the use of light hydrocarbons such as C.sub.1 through C.sub.4 for mixing with CO.sub.2 to create a conditional miscible transition zone which is injected ahead of a carbon dioxide flood.